Field
The present disclosure relates to techniques for detecting optical signals. More specifically, the present disclosure relates to an integrated photo-detector that includes a mirror that enhances absorption of an optical signal.
Related Art
Silicon photonics is a promising technology that can provide large communication bandwidth, low latency and low power consumption for inter-chip and intra-chip interconnect or link. A key component for use in inter-chip and intra-chip silicon-photonic interconnect is a photo-detector. For high-speed applications, the photo-detector needs to have high responsivity and high bandwidth. In general, an optical waveguide photo-detector geometry is preferred over a surface-illuminated photo-detector, because it can have a longer absorption length without the carrier-transit-time limiting the bandwidth. Optical waveguide photo-detectors have been demonstrated with responsivity up to 1 A/W (at a wavelength of 1550 nm) and a speed beyond 25 Gb/s.
However, chip-to-chip interconnects typically place more restrictions and requirements on photo-detectors. In particular, a CMOS-compatible process may not allow high-temperature growth of the photo-detector material. If a lower growth temperature is used, the absorption coefficient in the C-band (wavelengths between 1530 nm and 1565 nm) may be reduced because of the lack of tensile strain. As a consequence, a photo-detector may take longer absorption length to achieve high responsivity, which can result in a higher capacitance and a lower bandwidth. Furthermore, chip-to-chip interconnects usually require very low-power-consumption optical receivers, which requires the photo-detector capacitance be as small as possible (instead of just high bandwidth). For example, a 30 GHz photo-detector bandwidth can be achieved with 100 fF capacitance, but a low-power-consumption specification may require a photo-detector capacitance below 20 fF.
The relationship between the optical receiver power and the photo-detector capacitance can be understood as follows. The optical receiver in a silicon-photonic interconnect usually includes a photo-detector followed by a transimpedance amplifier. Furthermore, the transimpedance-amplifier signal-to-noise ratio may be maximized when its input capacitance (Ci) is roughly equal to the photo-detector capacitance (Cd). Because the transimpedance-amplifier power consumption is proportional to its size, and hence to its capacitance Ci, a smaller photo-detector capacitance results in reduced power consumption by the transimpedance amplifier, which is often a significant portion of the power consumed by the entire interconnect. In general, small photo-detector capacitance is desirable in high-speed interconnects, because the maximum signal-to-noise ratio is proportional to (BW2·Cd)−1, where BW is the bandwidth of the optical receiver.
However, high responsivity and low capacitance are usually conflicting requirements for photo-detectors, because the former requires longer optical waveguides to increase the absorption, whereas the latter requires the opposite. As a consequence, there is usually a difficult tradeoff when designing photo-detector absorption length.
Hence, what is needed is a photo-detector without the above-described problems.